Oxazolidinones are critical last-resort antimicrobials against severe infections caused by Gram-positive bacteria, whereas mobile oxazolidinone resistance genes (MORGs, including cfr, optrA, poxtA) severely compromise their clinical efficacy. To date, most surveillance and mechanistic studies focusing on MORGs have focused on aerobic pathogens, while anaerobic commensal and opportunistic bacteria are largely overlooked as hidden resistance reservoirs, leading to critical knowledge gaps under the One Health framework. Here, we identified six MORG-carrying anaerobic gut bacteria belonging to four distinct genera. Notably, this is the first report on the identification of cfr (B) in Gram-negative bacteria and optrA in three previously unreported host species. Genomic dissection further confirmed diverse mobile genetic elements driving horizontal dissemination of MORGs across phylogenetically distant anaerobes. Multiple additional resistance genes coexisted with MORGs in these gut anaerobes. Our findings demonstrate that intestinal anaerobic bacteria constitute a neglected critical reservoir for clinically vital MORGs, highlighting an urgent need for strengthened antimicrobial resistance surveillance of anaerobes across animal and human hosts.

Keywords: Gut anaerobes; antimicrobial resistance; mobile genetic elements; oxazolidinone resistance genes; oxazolidinones.